A stepper-type or step-and-scan-type projection aligner is used in the manufacture of semiconductor devices and the like to transfer a reticle pattern as a photo mask to each shot region on a wafer coated with a photoresist through a projection optical system.
The resolution of a projection optical system provided in the projection aligner increases as the exposure wavelength used becomes shorter and the numerical aperture of the projection optical system becomes greater. Therefore, the exposure wavelength which is a wavelength of radiation used in the projection aligner has been reduced in accordance with scaling down of integrated circuits year by year, and the numerical aperture of the projection optical system has been increased.
Depth of focus is as important as resolution. Resolution R and depth of focus δ are respectively shown by the following formulas,R=k1·λ/NA  (i)δ=k2·λ/NA2  (ii)wherein λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. When obtaining the same resolution R, a larger depth of focus δ is obtained by using radiation with a shorter wavelength.
A photoresist film is formed on the surface of an exposure target wafer, and the pattern is transferred to the photoresist film. In a common projection aligner, the space in which the wafer is placed is filled with air or nitrogen. When the space between the wafer and the lens of the projection aligner is filled with a medium having a refractive index of n, the resolution R and the depth of focus δ are shown by the following formulas.R=k1·(═/n)/NA  (iii)δ=k2·nλ/NA2  (iv)
For example, when water is used as the above medium in an ArF process, the resolution R is 69.4% (R=k1·(λ/1.44)/NA) and the depth of focus is 144% (δ=k2·1.44λ/NA2) in the case in which the photoresist is exposed through air or nitrogen, when the refractive index of light with a wavelength of 193 nm is n=1.44.
Such a projection exposure method to transfer detailed patterns by reducing the wavelength of emitted light is called a liquid immersion lithographic method and is regarded as an essential technique for miniaturizing lithography, particularly lithography of the order of several tens of nanometers. And the projection aligner for the method is also known (see patent document 1).
In the liquid immersion lithographic method using water as a medium of immersion, a photoresist film formed on a wafer and the lens of a projection aligner are respectively brought into contact with water. The water may permeate the photoresist film and reduce the photoresist resolution. In addition, the components forming the photoresist may be eluted into the water and may pollute the surface of the lens of the projection aligner.
A method of forming an upper layer film on a photoresist film may be used in order to block contact between the photoresist film and the medium such as water. Such an upper layer film must possess sufficient transparency to radiation with a wavelength used in liquid immersion lithography, must form a protective film on a photoresist film while causing almost no intermixing with the photoresist film, must be stably maintained without being dissolved in the medium such as water during liquid immersion lithography, and must be easily dissolved in a developer such as an alkaline solution.    [Patent Document 1] JP-A-11-176727